Method for clamping a knife in a disk chipper and a knife clamp for a knife assembly

ABSTRACT

The invention concerns a method for clamping a knife of a disc chipper, wherein the knife is pressed from the knife disc via knife a clamp against a wear plate in a diagonal position (a) with respect to the plane of the knife disc with a compressive force (N) substantially parallel with the axis of the knife disc. According to the invention, the compensating force of the force directed to the knife clamp, caused by the diagonal position (a) of the knife and transversal to the compressive force (N) is exerted to the wear plate.

The present invention concerns a method for clamping a knife of a discchipper against a wear plate by means of a knife press between the wearplate and the knife disc. The invention also concerns a knife clamp,that is pressed by a device supported from the knife disc and that issupported to the knife and the wear plate.

Disc chippers are generally used in the wood processing industry forchipping wood prior to the further processing. The rotating disc of achipper is equipped with knives attached evenly distributed theretocutting chips from a log against a counter knife. The knives aregenerally attached to the disc through separate knife presses that arelocked in position generally by means of a compressive force provided bybolts. The knife press by means of which the knife is pressed in itsposition is in this publication referred to as knife clamp.

When attaching a knife in its position in the knife disc according to amethod of prior art, the knife clamp and the knife are pressed withbolts against the wear plate. The method is described e.g. in thepublication U.S. Pat. No. 6,056,030. The bolts are parallel with theaxis of the knife disc. The bolts have their support from the knife discand move the knife clamp and the knife in the axial direction of theknife disc, away from the knife disc towards the inclined surface of thewear plate, against which the knife is positioned. This positioningsurface of the knife in the wear plate deviates due to its inclinationfrom the direction perpendicular to the pressing motion of the clamp,whereby the knife clamp and the knife are subject to forces from thewear plate, that tend to move the knife clamp aside from the directionof the desired pressing motion. Against this transversal motion theknife clamp is supported from the knife disc near the compressing bolt.A friction force is generated to the point of support resisting themovement of the knife clamp during the compressing process.

The method in accordance with the present invention is characterized in,that the point of support preventing the transversal movement of theknife clamp is positioned in the wear plate on the area of the planebetween the knife and the knife clamp, said area facing the knife,whereby the transversal force required for attaching the knife increasesthe compressive force between the knife clamp and the knife. The knifeclamp in accordance with the invention substantially takes the supportof the knife clamp against the transversal movement from the wear plateby means of a bracket of the knife clamp. Due to the method, thecompressive force of the knife clamp can be provided also with othermethods than bolt attachment, because a supporting surface of the knifedisc under the knife clamp is not essential.

The invention and its details will now be described in more detail withreference to the enclosed drawings, where

FIG. 1 shows a traditional knife assembly and knife clamp of a discchipper,

FIG. 2 shows the forces directed to the knife clamp,

FIG. 3 shows a knife clamp according to the present invention,

FIG. 4 shows forces directed to the knife clamp according to the presentinvention, and

FIG. 5 shows a preferred construction according to the presentinvention.

FIG. 1 shows a generally used knife assembly. Knife 1 is attached in itsplace by pressing the knife by means of a knife clamp 2 against a wearplate 3. Bolts for attaching the wear plate to the knife disc 4 is notshown in the figure. The knife clamp 2 is pressed in the direction ofarrow N against the knife 1 by means of a bolt 5. Bolt 5 is supportedfrom the knife disc 4 by means of a hardened threaded bushing 6. Thesurface 7 of the knife clamp placed against the knife 1 is formedslightly concave, so that the knife clamp is supported against the knifeat points 8 and 9. Thus, the knife 1 has the best possible support fromthe knife clamp 2 despite of small inaccuracies in manufacturing. Themost important thing is to the have the knife supported in the bestpossible way near the point of the knife by means of surface 8.

It has been noticed in the practice that the point portion 11 of thewear plate 3 slightly yields due to the force N. Thus, also the knife 1and the knife clamp 2 must turn from 0.2 to 0.3 mm at the point portion12 thereof. Turning of the knife clamp and tensioning of the bolts 5 arecontributed by the rounded ends 13 of the bolts.

Against the direction perpendicular to the pressing motion, the knifeclamp is supported by means of the surface 14. The surface 14 issupported against the counter surface 15 in the knife disc and afriction force caused by the supporting force is generated between thesesurfaces.

During the chipping process the point 11 of the wear plate is in a smallcontinuous bending motion and the size of said friction force variescontinuously. In praxis this variation causes a pulsating effect to thecompressive force of the point 8 of the knife clamp. Function of theknife clamp is to keep the knives firmly attached to the chipper andanother main function is to maintain an adequate compressive force onthe area of the point 8. Decreasing of said compressive force issubstantially influenced not only by the bending of the wear plate butalso the force turning the knife clamp caused by the compressive forcebetween the surfaces 14 and 15, said force being dependent on the heighth and the angle α of the clamp, as shown in FIG. 1. The height of theclamp is dependent on the adequate stiffness of the clamp and thedistance between the screws 5. Angle α of the knife is in general about36-40 degrees and it is influenced by the sharpening angle of the pointof the knife.

Referring to FIG. 2, the arrows describe forces exerted by the knifeclamp 2 to the other parts of the knife disc. For simplicity of theforce figure, the concavity of the surface 7 of the knife clamp againstthe knife is not taken into account. The compressive force caused by thebolt is P₁ (=N). The correspondingly big supporting force P₁′ exerted tothe knife 1 is divided into component P₂ effecting against the knife andcomponent P₃+P_(μ) effecting in the direction of the knife. Force P₃parallel with the surface of the knife, which is a residual force takinginto account the friction force caused by P₂, tends to move the knifeclamp from its place. The size of the force is dependent on the frictionbetween the knife and the knife clamp. The surface of the knife clampfacing against the knife of the knife clamp is effected by the force P₃coming from the knife, and force P₃′ affecting the knife disc is formedto the supporting surface 14. Forces P₃μ′ and P₊ parallel with thesurface 14 are formed thereby.

Force P₂ effects in perpendicular direction to the surface of the knifeand it is divided into counter forces P₄ and P₅, which are located ontheir respective areas of influence 8 and 9. Most important is that theforce P₄ maintains adequate. Distance of the above mentioned pair offorces is c and the size of this distance varies to some extentaccording to inaccuracies of the surfaces of the knife 1 and the knifeclamp 2. Force P₃ moving aside the knife clamp from the direction ofeffect of force P₁ gives a decreasing effect (P₄) and an increasingeffect (P₅) according to formula (b/c) P₃ to the pare of forces P₄-P₅.This results in additional need of force P₁, when a certain level offorce P₄ is needed.

A bigger height h (FIG. 1) also increases the dimension b and requiresincreasing of the clamping force P₁. Harmful effect of this problemincreases during the operation of the chipper, whereby the vibrationdecreases the influence of the friction and the compressive force of thearea 8 can decrease as much as 50%. A minor effect increasing the forceP₁ comes from force P₊ according to formula (a/c) P₊. This increasingportion of the main force P₁ ranges from 3 to 10%. When the knife 1 getsnarrower due wearing, the dimension a is 0 and there is no favourableeffect P₊.

A knife clamp 2′ according to the invention is shown in FIG. 3. Theknife clamp 2′ is supported against the perpendicular direction parallelwith the compressive motion N by means of the surface 17 of the bracket16. Otherwise the knife clamp 2′ corresponds to that shown in FIG. 1.Counter surface for the bracket 16 is the surface 18 of the groove 20 inthe wear plate 3. The system according to the present invention has thatconsiderable benefit that no supporting surface 15 is needed and theknife clamp according to FIG. 3 can also be used with other attachingmethods than bolts.

In the knife clamping system of FIG. 3 according to the presentinvention, the support of the transversal force effected by the clampingforce N of the knife clamp has been moved over to the other side of thesurface 30 between the knife 1 and the knife clamp 2′ compared with thesystem shown in FIG. 1. The force pattern of the construction of FIG. 3shown in FIG. 4 shows that correspondingly force P₃ parallel with thesurface of the knife as well as a corresponding supporting force P₃′ aregenerated in this construction to the surface 17. The force increasingthe force P₂ against the knife can be calculated from the FIG. 4 byformula (d/e) P₃′. As it is evident from said formula, that a longsurface 7′ against the knife is unfavourable with respect to theincreasing of force P₂. A wide knife 1 according to FIG. 3, however,requires a wide knife clamp 2′.

The advantages of the present invention become evident from theconstruction in accordance with FIG. 5, where the knife 24 is led inposition through the groove 25 by means of a projection 22 of the knifeclamp 23. Turn knife 24 equipped with two points is much narrower thanthe knife of FIGS. 1 and 3 and requires thereby a better support.Thereby the supporting of the knife 24 is a much more demanding task andthe knife clamp according to the invention and FIG. 5, where therelation of dimensions f/g is bigger than the relation d/e according toFIG. 3 is a suitable solution for this purpose. An adequate strength isalso reached without any problems, because height h′ does not matter.

A wide knife as shown in FIG. 3 requires two supporting regions 8′ and9′ between the knife clamp and the knife. There the bracket 16 cannot beused for supporting the force P₂, because a three-point or three-areasupporting is not successful due to fabrication inaccuracies. The knifeclamp according to FIG. 5, instead, forms only one supporting area 28against the knife from the bottom of the groove 25. Another balancingsupport is provided by the bottom surface 26 of the groove 20′, againstwhich the corresponding surface 21 of the bracket 16′ is supported,whereby bracket 16′ receives the force P₃′ and a part of the force P₂.Thereby an adequate clamping of the knife can be achieved with areasonable force N.

The arrangement in accordance with FIG. 5 has an additional advantagetherein that the knife disc 4 has a surface 29 with no groove. Thismeans considerable savings in costs of machining.

Oversized compressive forces require a large number of clamping screwsand big forces result in durability problems of threads and bending ofthe wear plate 3 as well as in permanent deformation, which causesfunctional disturbances and need of changing components. By means of amethod and a knife clamp in accordance with the present invention, thedisadvantages mentioned above can be avoided.

1. A method for clamping a knife (1%) of a disc chipper, wherein theknife is pressed from the knife disc (4) via knife clamp (2′, 23)against a wear plate (3) in a diagonal position (α) with respect to theplane of the knife disc with a compressive force (N) substantiallyparallel with the axis of the knife disc, wherein a compensating forceof the force directed to the knife clamp, caused by the diagonalposition (α) of the knife and transversal to the compressive force (N),is exerted to the wear plate.
 2. A method in accordance with claim 1,wherein the compensating force of the transversal force is exerted tothe wear plate (3) in parallel direction with the knife (1), facing thewear plate with respect to the plane between the knife and the wearplate, whereby the compensating force brings about a moment that turnsthe knife clamp and increasing of the compressive force (P₄) on theregion (8) of the knife point.
 3. A clamping arrangement for a knife ofa disc chipper comprising a knife disc (4), a wear plate (3), a knife(1), compressing means (5) and a knife clamp (2′ or 23), wherein alateral force (P₃) is generated, said force deviating by angle (α) fromthe perpendicular direction to the compressive force (N), wherein thereis a bracket (16) in the knife clamp (2′ or 23), the surface (17) ofwhich receives the generated force (P₃) by supporting itself against asurface (18) of the groove (20) of the wear plate (3).
 4. A knife clamp(23) for the clamping arrangement of claim 3, wherein the upper surface(21) of the bracket (16′) is supported to the bottom surface (26) of thegroove (20′) of the wear plate (3).
 5. A knife clamp in accordance withclaim 4, wherein the knife clamp (23) comprises a projection (22)guiding the knife (24) from a groove (25).